The present invention relates to a single station thermo-forming machine for forming three-dimensional work pieces, which have extensive surfaces but are relatively thin and of even thickness, from flat sheets or plates of thermoplastic material. The invention relates more particularly to a thermo-forming machine for vacuum forming.
In the prior art, an operator loads a sheet of thermoplastic material to be formed into a conventional single-station thermo-forming machine where a radiant heating panel heats it to a temperature at which the material is in a resilient plastic state. Once it has been heated and softened, the sheet is stretched by compressed air supplied from the base of the machine by means of a solenoid valve. A mould is raised and a vacuum is used to cause the stretched sheet to adhere to it. The thermo-formed work piece is then cooled using air and vaporised water and then unloaded by the operator. The production capacity of a standard single station block thermo-forming machine is in the region of 40 work pieces per hour if the sheets are around 4 mm thick. Machines of this type are subject to the following limitations:    heating is provided by only two heating panels, an upper one and a lower one;    it is impossible to heat the sheet and carry out the thermo-forming simultaneously;    only about 60% of the overall electric power supplied to the machine is actually used since during the thermo-forming step the radiant panels are powered but are not heating a sheet.
Thermo-forming machines with a very high productivity, known as in-line thermo-forming machines are also known in the industry. These machines are made up of five stations:    a first station where the cold sheet is loaded onto a conveyor system;    a second, pre-heating, station with an upper and a lower panel for pre-heating the sheet;    a third, final heating station with an upper panel and a lower panel;    a fourth station for vacuum forming the sheet in a mould; and    a fifth station for trimming and unloading the formed work piece.
Although in-line thermo-forming machines have a high productivity and make use of 100% of the electricity supplied for heating the sheets, they are expensive, take up a lot of space, require several operators and also involve problems in transporting the sheets, which are conveyed by a chain and roller system which grips two opposite edges of the sheets.